1. Field of the Invention
The present invention relates to a projection display device that enlarges and projects an image on an imager onto a projection plane, and more specifically is suitably used for a projection display device that projects light onto a projection plane in an oblique direction.
2. Description of the Related Art
At present, there are commercialized and widely spread projection display devices (hereinafter, referred to as “projectors”) that enlarge and project an image on an imager (such as a liquid crystal panel) onto a projection plane (such as a screen). For this kind of projectors, various techniques for shortening a throw distance have been proposed. The term “throw distance” herein refers to a distance from a projection aperture in a projector or a terminal optical component in a projection optical system to a plane including a projection plane. If the throw distance is short, projected light is less prone to be cut off by some obstacle, thereby increasing usability of the projector and convenience of a user.
As a technique for shortening a throw distance, there is a possible configuration of a projector in which an optical system is made wide-angle and a travel direction of projected light is inclined with respect to a light axis of the projection optical system. In this arrangement, for example, a wide-angle lens with a large angle of view is used as a projection optical system, and an imager and a screen are shifted so as to be mutually opposed with respect to the light axis of the projection optical system. This accomplishes shortening of the throw distance.
Another arrangement uses a refracting optical system and a reflecting optical system as a projection optical system. In this arrangement, an image on an imager is formed as an intermediate image between the refracting optical system and the reflecting optical system. The intermediate image is enlarged and projected onto the screen in an oblique direction by the reflecting optical system (curved mirror). This accomplishes shortening of the throw distance.
On a projector with such an oblique-projection arrangement, a size of a projected image (hereinafter, referred to as “projection size”) can be adjusted by changing the throw distance as appropriate. For example, the projection size can be decreased by making the projector closer to the screen to shorten the throw distance. In addition, the projection size can be increased by making the projector away from the screen to lengthen the throw distance.
When the throw distance is changed in such a manner as stated above, the projected image is brought into an off-focus state, and it is thus necessary to make a focus adjustment as needed. Such a focus adjustment can be made with relatively simple operations of a focus adjustment mechanism provided in the projection optical system. However, in such an arrangement in which projected light is obliquely projected onto the screen at a wide angle as described above, the position of the projected image on the projection plane shifts significantly upward or downward with a change in throw distance. Accordingly, in changing the projection size during usage of the projector, it is necessary to make a focus adjustment and a position correction in a rapid manner to the projected image while making a change to the throw distance.
For general use, when installing a projector that projects obliquely an image onto a screen (projection plane) at a wide angle, height of the screen is adjusted or height of the projector is adjusted using a stand or the like as appropriate, so as to project the projected image onto the screen in a desired position (e.g. the center). Accordingly, when making a position adjustment to the projected image with a change in projection size as stated above, it is necessary to adjust screen height or projector height, and such adjustments involve significantly complicated tasks.
Further, in a projector with a refracting optical system and a reflecting optical system as a projection optical system, the projection optical system becomes large in size and weight due to the additional reflecting optical system. Additionally, in this arrangement, since the reflecting optical system (curved mirror) forms a wide-angle structure, the position of a projected image varies greatly even with a slight change in inclination of the projector main body. Therefore, significantly delicate tasks are needed for making a position adjustment to a projected image. From such factors as stated above, it is understood that a projector with a reflecting optical system requires considerable efforts and complicated tasks in making a position adjustment to a projected image. To simplify such adjustment tasks, a separate mechanism may be added to make a height adjustment to the projection optical system. However, since the projection optical system is large-sized and high-weight as stated above, a significantly large-scale mechanism is required to displace the overall projection optical system including the refracting optical system and the reflecting optical system.